Ink-jet printers which print by injecting ink supplied to a printhead toward a printing medium (printing sheet) by heating or vibrations are the mainstream among current serial printers because of high image quality and low cost.
Such ink-jet printer uses a stepping motor and servo-controlled DC servomotor in order to serially scan a carrier which supports an ink-jet head serving as a printhead.
Recently, in order to reduce printing noise and increase the printing resolution, many printers adopt a DC servomotor which supports a magnetic or optical encoder so as to scan it together with a carrier and undergoes servo (feedback) control so as to converge the carrier to a target speed or position on the basis of speed information or position information obtained from the encoder.
In servo-controlling a carrier by using a DC motor, PID control is generally employed.
PID control is a control method of determining a target position at each time and a target speed at each time, calculating a proportional element, integral element, and differential element from the deviations between the target position and target speed at each time, and speed information and position information actually obtained from the encoder in accordance with the time schedule, changing energy (e.g., PWM-controlled DC current value) to be applied to the DC servomotor, and smoothly driving the carrier with small noise.
Servo control has conventionally been executed by always referring to speed information detected by an encoder. When the carrier starts operation, the carrier has rarely gained a speed yet, but a speed (speed higher than an actual one) different from an actual carrier speed may be instantaneously detected due to carrier vibrations or the like.
In this case, speed information much higher than the target speed is referred to in servo control immediately after the start of operation, and the output is narrowed down in the next servo control. The output hardly increases, and a long time is taken until energy for actually activating the carrier. In the worst case, an error determination condition is met, and an error stop occurs.